Human civilisation may become completely unsustainable unless we slow the destruction of Earth’s plant life, a new study has warned.

“You can think of the Earth like a battery that has been charged very slowly over billions of years,” said lead author John Schramski, associate professor in University of Georgia’s College of Engineering.

“The sun’s energy is stored in plants and fossil fuels, but humans are draining energy much faster than it can be replenished,” he said.

Earth was once a barren landscape devoid of life, he said, and it was only after billions of years that simple organisms evolved the ability to transform the sun’s light into energy.

This eventually led to an explosion of plant and animal life that bathed the planet with lush forests and extraordinarily diverse ecosystems, researchers said.

Chemical energy is stored in plants, or biomass, which is used for food and fuel, but which is also destroyed to make room for agriculture and expanding cities.

Scientists estimate that the Earth contained approximately 1,000 billion tonnes of carbon in living biomass 2,000 years ago.

Since that time, humans have reduced that amount by almost half. It is estimated that just over 10 per cent of that biomass was destroyed in just the last century.

“If we don’t reverse this trend, we’ll eventually reach a point where the biomass battery discharges to a level at which Earth can no longer sustain us,” Schramski said.

Working with James H Brown from the University of New Mexico, Schramski and University of Georgia’s David Gattie showed that the vast majority of losses come from deforestation, hastened by the advent of large-scale mechanised farming and the need to feed a rapidly growing population.

As more biomass is destroyed, the planet has less stored energy, which it needs to maintain Earth’s complex food webs and biogeochemical balances.

“As the planet becomes less hospitable and more people depend on fewer available energy options, their standard of living and very survival will become increasingly vulnerable to fluctuations, such as droughts, disease epidemics and social unrest,” Schramski said.

If human beings do not go extinct, and biomass drops below sustainable thresholds, the population will decline drastically, and people will be forced to return to life as hunter-gatherers or simple horticulturalists, according to the researchers.

The study was published in the journal Proceedings of the National Academy of Sciences.